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Case Study: Sulphur Soil Testing by Zone


There are several different ways to soil sample a field: random, benchmark, topographic and grid, to name a few. At CropPro, we sample by our SWAT MAP zones. We soil test 5 different zones (zone 1-2 together, zone 3-4, etc.), which allows us to group similar soil characteristics together and therefore, similar fertilizer responses together. In this study, we sampled 25 locations (5 locations per zone, 3 probes at each location) within a field.

Figure 1. 3D SWAT MAP of field sampled in this case study.

Sulphur Results

Some people within the industry claim that you cannot accurately test for sulphur “so how can you VR it?” Our response is: if you soil test it by zone 99% of the time this is what you will see (Figure 2). Figure 2 shows that the hills (zones 1-4) are deficient and the depressions (zones 7-10) are sufficient.


Sulphate is mobile within the soil and moves with water throughout the field. As the water moves from the hilltops into the depressions, it is moving sulphate with it. Differences in soil organic matter within the field also play a huge role in sulphate levels. Soil microbes that decompose organic matter and plant residues release sulphate. Therefore, the depressions that have higher organic matter will have higher levels of sulphate than the low organic matter hilltops. The combination of higher organic matter and water accumulation are why zones 7-10 on our SWAT MAPS will have higher levels of sulphate 99% of the time.


Figure 2. Sulphate (lbs/ac) at 0-8” depth from 25 locations within field tested individually and separated by zone.

Analyzing the results from Figure 2, our recommendation on this field would be to apply high rates of sulphur in zones 1-4, moderate rates in zones 5-6 and cut back rates significantly in zones 7-10.


Summary

Soil sampling by zone gives us confidence in our fertilizer recommendations. Figure 2 shows that the variability of sulphate levels in a field can be grouped together when sampled by zone.


Brandon Smith

Precision Agronomist

brandon@croprecords.com

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